I don't know about a designer breed but growing up in Southwestern Alabama we crossed Catahoulas with Mutt Bulldogs. We called any Bully mix "Bull Dogs" and my Uncle had three or four big strong males that we crossed with big Catahoula Females. These were some tough dogs, man they could hunt wild hogs, and I heard that they would hunt bear too. The bulldog gave them width and strength in their chests and shoulders. It didn't change their bark or tenacity though. We weren't selling dogs or anything like that these were just for us. We had about 10 dogs all together with the two litters we had. My family still had some pups of those originally dogs up until about three years ago when my cousin stopped hunting. His last puppy would've been about 11 years old when it died. It was feisty to the end.

I LOVE Catahoula leopard dogs and have read up on the Catahoula bulldog as well a while back.
As for message boards about them... not likely any. You could look for bully boards or the like, or just hang around here

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Courage is being scared to death and saddling up anyways. ~John Wayne